Portable devices for producing AC electric power in the field which combine a relatively small engine generator are well known in the art and are utilized for leisure, outdoor construction work and as a power source for emergency use.
In an engine powered electric generator ("engine generator") of this general type, it is desirable to provide the load with an AC output having both a stable output frequency and a stable output voltage. Such stable output frequency and voltage must be maintained over a wide range of possible loads. In typical use, the load on an engine generator may vary over time as the load is cycled through different duty levels.
Generally, a synchronous AC generator has been employed in an engine generator. The relation of n=120 f/p is well known in the art where "n" is the rotational rate of the engine in revolutions per minute, "f" is the electrical output frequency of the AC generator in Hertz, and "p" is the number of magnetic poles of the generator. Bipolar or tetrapolar generators are commonly used. For example, the speed of rotation of the engine is kept at approximately 3000 rpm (or approximately 3600 rpm) in a bipolar type generator in order to obtain a stable AC output of 50 Hz or 60 Hz, respectively. As a result, in AC generators where output frequency is determined by the rotational speed of the generator, a larger generator is required to obtain a larger output. However, under these conditions, the rotational speed of the engine may be comparatively low and its operational efficiency may not be optimized. Furthermore, the rotational speed of the engine tends to fluctuate due to load variations or engine pulsation making it difficult to maintain a constant electrical output frequency.
To cope with this problem, some recent engine generators have been miniaturized and their operational efficiency enhanced by permitting higher rotational velocity. To stabilize the electrical output, the output of the AC generator is rectified to Direct Current ("DC"), and then converted to AC again with a precise preset frequency independent of the rotational velocity of the engine by means of inverter circuitry. Simultaneously, output voltage is automatically regulated by varying the width of an inverter driving signal with the aid of a feedback signal derived from the output voltage. This type of engine generator is disclosed in the specification of Japanese Laid-Open Patent Application No. 82098/1985.
In the prior art engine generator in which the output voltage is stablized by the above-described feedback control, however, the circuits necessary to achieve such control are complex and the voltage regulation does not account for fluctuations in output voltage due to pulsation of the engine or to a sharp increase in output voltage. Such engine pulsation or sharp increase in output voltage can occur due to an increase in the velocity of rotation of the generator. This is, in turn, caused by the engine's governor when the output load changes abruptly.
Furthermore, in the conventional engine generator which utilizes the aforementioned inverter, if the load is connected to the output of the inverter while the engine generator is in a transient state of increasing output voltage, as for example, at start up, a semiconductor switch element such as a transistor at an output stage of the inverter may suffer a shortage of bias current potentially causing improper output. Moreover, if a commutation command or signal with a predetermined frequency is generated by applying a voltage obtained from the rectified output of the generator to drive the inverter, the frequency of the commutation signal sent from an oscillator circuit while the engine generator output voltage is in a transient state of increasing output voltage may become unstable in some cases. Such problems may also occur when the output voltage falls due to stoppage of the engine generator. Accordingly, an improved power regulating system for a portable engine powered electrical generator would be desirable.